Nowadays, graphite, a form of carbon, is the most important anode materials for lithium ion batteries which are commonly used in cell-phones, notebooks, digital cameras, power tools, etc. As the automotive industry turns to electric vehicles, the current lithium ion battery system and technology face many new challenges, such as low specific energy density, safety problem of battery packs, high price of unit energy of Wh, etc. The main reason why the current lithium ion batteries have a relatively low specific energy density relates to the type of the anode used in the batteries. The graphite anodes currently used have a low capacity (theoretically 372 mAh/g) for storing lithium ions, and this leads to a problem of a low overall storage capacity of the batteries. New materials, methods and technologies shall be developed for lithium ion battery before it goes into mass use in Pure EVs and Plug-in EVs.
Silicon is one of the most promising candidate anode materials for lithium ion batteries because of its highest theoretical specific capacity (4200 mAh/g) and low discharge potential. Now researchers have realized that the anode capacity can be increased by many times, if graphite is replaced by silicon composites. By far, the problem of this approach lies in that silicon is easily cracked and pulverized when it is loaded with lithium ions during the charge-discharge cycles. So the cycling ability of the battery will be very poor.
The use of Si/C composites has been investigated for many years to overcome the limitation of pure Si powder. Conventional composites, which are commonly prepared by pyrolysis, mechanical mixing and milling, or a certain combination of these two, consist of Si particles embedded into a dense carbon matrix. However, a large volume change in Si during Li insertion can be accommodated by carbon only to a limited degree, and thus, only a limited enhancement of stability and capacity can be achieved.
Porous structure is an effective approach to accommodate the volume change. Zheng, Y et al., Electrochim. Acta, 2007. 52(19): p. 5863-5867 discloses a nano-porous silicon composite incorporated with graphite and pyrolyzed carbon, which is prepared by a template method including two-step ball-milling followed by etching with hydrochloric acid. The initial specific charge capacity is 649 mAh/g, the initial specific discharge capacity is 1019 mAh/g, and the efficiency is 64%. Said composite shows a good cycling stability and no capacity loss after up to 120 cycles. This is attributed to the nanosized pores in Si particles. However, the reversible capacity of said composite is relatively low.
CN 1761089 A discloses a Si/C/graphite composite anode material for lithium ion battery, in which the content of elementary silicon is 10-80 wt. %, the content of graphite is 10-60 wt. %, and the rest is amorphous carbon. The initial specific charge capacity is 1115 mAh/g, the initial specific discharge capacity is 1365 mAh/g, and the efficiency is 82%. At the 10th cycle, the specific charge capacity is 784 mAh/g, the specific discharge capacity is 838 mAh/g, and the capacity retention is 70%.